Hippos mate in the water. The male hippopotamus rides on the female hippopotamus from behind. The buoyancy of water can help female hippos avoid being hurt by the weight of male hippos during mating. The male hippopotamus can weigh twice as much as the female hippopotamus, and with four short legs, it can't be supported on the back of the female hippopotamus at all. The buoyancy of water can reduce the burden of weight.

The complete pregnancy of a female hippopotamus is 8 months. They only give birth to one at a time. Before the baby hippo is born, the mother hippo will leave other hippos and be alone. She will give birth to cubs underwater or in water. The female hippopotamus will live with the newborn baby for 3 to 4 weeks, and then return to the hippopotamus group together. The time spent alone between mother and son ties them together. After the mother and son return to the group, all the female hippos will help raise the baby hippos. Although the female hippopotamus will do most of the child-rearing work, all the female hippopotamus will work together in Qi Xin to protect their baby hippopotamus from danger.

Hippos are polygamous (whatever), which means they will have more than one spouse. Every time hippos mate, the object may be different. Female hippos reach sexual maturity at about 3 years old, but usually they don't mate until 7 to 8 years old. Female hippos usually mate every two to three years. Male hippos reach sexual maturity when they are about 5 years old, but they usually wait until they are bigger or more dominant before they have their turn to mate. During the breeding season, strong males will drive away less strong males and enjoy the female hippo exclusively. In most cases, only the dominant males have a chance to mate.